Most professional mobile radiocommunications systems use the principle of “trunking”, that is to say the network possesses an infrastructure which manages the use of the radio resources, throughout the duration of the communications. The network allocates the resources to the communications and controls the access of the terminals to the air interface.
Systems without “trunking” also exist, in which mobile terminals can communicate in direct mode or in relayed mode, that is to say by way of repeaters. In general, the voice communications are of the walkie-talkie type, with channel access on an alternate turns basis. The terminal listens to the frequency of the channel so as to determine whether it has the right to transmit when the user actuates a PTT (“push-to-talk”) button with which the terminal is provided. The right to transmit is granted if the channel is free or busy with a communication over which the terminal has priority.
In this latter type of system, the call with off-hook services (FOACSU: “full off-air call set-up”) of telephone type are not supported. Specifically, in the absence of a base station or repeater managing the connection and access to the radio channel, a communication can be interrupted by an external user, during a silence on the channel due to the halting of voice transmission.
An aim of the present invention is to propose a way of supporting the services of call with off-hook type in radiocommunications systems without “trunking”.